Hot stamping is one method of forming by which a steel sheet, heated at a temperature not lower than the Ac3 point, is pressed using metal dies, and then quenched by rapidly cooling the steel sheet in the dies. According to the hot stamping, the steel sheet may be improved in the strength, and may be ensured a desired level of geometrical stability. Heating of the steel sheet, at the primary stage of the hot stamping, is often performed by furnace heating, near-infrared heating, far-infrared heating, induction heating, direct energizing heating or the like.
If the material to be hot-stamped is a galvanized steel sheet, the galvanized steel sheet is heated in the heating process to a temperature not lower than the point Ac3, and below the boiling point of zinc, practically at 900° C. or lower. When heated up to this temperature range, plated zinc turns into a molten state, and thereby liquid phase diffusion of iron into the molten zinc proceeds. Accordingly, in an intermediate cooling process between the end of heating and the start of pressing, iron concentration in the molten zinc increases to 15% to 30%, and alloying between zinc and iron proceeds when the steel sheet is cooled down below 782° C. Γ phase appears as a result of alloying.
In the hot stamping of a galvanized steel sheet, timing of pressing is important for the reason described below. If the galvanized steel sheet is pressed before or immediately after the start of alloying reaction, the steel will cause embrittlement cracking at the grain boundary due to an unalloyed portion of the molten zinc, only to produce defective products. Even if the embrittlement cracking fortunately does not occur at the grain boundary of steel, the molten zinc will adhere on the inner surfaces of the dies, so that the dies have to be cleaned up quite often. This also decreases the zinc content in the surficial portion of products, degrades the corrosion resistance of the product, and thereby causes problems in performance of the resultant components. In the hot stamping of the galvanized steel sheet, it is therefore desirable to press the steel sheet after the alloying reaction is completed in the intermediate cooling process.
It is not, however, easy to start the pressing timely by appropriately controlling the heating process and the intermediate cooling process, since the galvanized steel sheet is now broadly diversified and include those with various types of plating, plate weight, sheet thickness and size. In short, although the conventional process control has been empirically based on heating time, heating temperature, intermediate cooling time, and starting temperature of pressing, it is difficult to exactly determine the end point of the alloying reaction.
One possible method for detecting the end point of the alloying reaction is to observe the surface of the galvanized steel sheet by X-ray diffractometry, and to detect the end point based on the result. The method, however, needs a large scale apparatus and pushes up the cost of equipment. Another problem is that detection in such high temperature state is difficult. Still another problem is that visual observation may vary largely from person to person, particularly under such high temperature condition, and this may inhibit stable control.
Patent Document 1 discloses a method of heating the galvanized steel sheet up to 800° C. to 950° C. in a heating furnace, followed by rapid cooling down to 500° C. to 730° C. in a quenching equipment, and pressing. The method is, however, hardly applicable to ordinary hot stamping of galvanized steel sheet, since the method is specialized for the purpose of improving corrosion resistance and fatigue resistance, and in need of quenching equipment.
On the other hand, Patent Document 2 describes a method of observing the degree of proceeding of the alloying reaction in a Fe—Zn system based on spectral emissivity, in the process of manufacturing of an alloyed galvanized steel sheet. The temperature range in which the observation takes place according to the method described in Patent Document 2 is, however, extremely lower than the temperature range in which the hot stamping takes place. It is therefore impossible for the method described in Patent Document 2 to detect the state of surface of the galvanized steel sheet, in the intermediate cooling process of hot stamping.